Clang Error on “Potential null dereference.”

The way to do what’s expected is shown in listing 3-5 in that document. With your example code:

+ (NSString *)checkForLength: (NSString *)theString error: (NSError **)error {
    BOOL hasLength = ([theString length] > 0);
    if (hasLength) return theString;
    else {
        if (error != NULL) *error = [NSError errorWithDomain:@"ErrorDomain" code:hasLength userInfo:nil];
        return nil;
    }
}